st: Heckman

Hi, Stata gurus:
I have a question about the Heckman 2-stage estimation:
consider the model:
Y=X*b+e1
I=Z*gamma+e2 and I*=1 if I>0
Usually, the Heckman command gives the regression coefficients b when I>=0
or I*=1;
1. is there a command for estimating Y at the mean value of the independent
varibales? like the mfx command default at mean value?
2. What should I do if I want to get the the coefficients b when I<0 or
I*=0? Is there a command for this?
Thanks a lot.
Gang
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